The history of luaTEX 2006–2009 / v 0.50 - Pragma ADE

XXVII Virtual Reality
When a font lacks glyphs we can add new ones by making the font virtual. A virtual font
has virtual glyphs: instead of a reference to a slot in the current font, such a glyph refers
to a slot in another font, or it combines several glyphs into one, or it just contains code
that ends up in the result (for instance a sequence of pdf commands that describes the
shape). For TEX a character and its dimensions are what matters and what ends up in the
result is mostly a matter for the backend. In LuaTEX the backend is integrated but even
then during the typesetting process only the characteristics of a glyph are used and not
the shape.
In ConTEXt we have a feature called ‘compose’ which extends the font with extra characters
and constructs its representation from those of other characters.
\definefontfeature
[composes]
[kern=yes,ligatures=yes,compose=yes]
When this feature is applied, ConTEXt will try to ll in the gaps in the Unicode vector of
the font based on for instance (de)composition information. Of course this has some limitations.
For instance OpenType fonts can ships with features, like smallcaps. Currently
we ignore this property when we add composed characters. Technically it is no big deal
to add variants but we simply didn't do it yet at the time of this writing. After all, such
fallbacks can best be avoided by using proper fonts.
Our ConTEXt MkIV wishlist mentions a mechanism for combining fonts into one font. For
this we can use virtual fonts and the machinery for that is in available in Lua code. However
such a mechanism will be used for more drastic completion of a font than the compose
feature. For instance, often Chinese fonts lack proper Latin glyphs and vise versa.
But when we combine such fonts we really do want to keep OpenType features working
and so we cannot use virtual fonts (unless we start merging features which can become
really messy and runtime consuming).
There is a relative simple solution using real fonts that kind of behave like virtual ones:
virtual real fonts. The trick is in the fact that TEX permits access to characters not present
in the font. Say that we have
and that slot 124 has no glyph. In that case TEX just inserts a glyph node with a reference
to the current font and this character. Of course, when we let TEX carry on, at some point
it will need glyph properties like the width, height and/or depth. And in the backend,
Virtual Reality 233